The crossover element itself is a huge trope. Combining the unique worlds of Naruto, Highschool DxD, and the concept of ghouls allows for all kinds of interesting scenarios. Such as when the different magic systems and ninja techniques interact. For instance, a character might try to use a jutsu in the world of Highschool DxD or combine a devil's power with a ghoul's ability.
Another common trope is the fish - out - of - water situation. When a character from one of the universes is suddenly thrust into another, like a Naruto character in the Highschool DxD world or vice versa while also dealing with the presence of ghouls. This creates a lot of comical and also tense moments as they struggle to adapt to new rules, cultures, and threats.
One popular trope is the 'Fish out of Water' scenario. For example, a character from Prototype suddenly finding themselves in the Highschool DxD world full of devils, angels, and other supernatural beings. It creates a lot of comical and interesting situations as they try to adapt.
A less common but still interesting trope is the 'reversal of roles'. In some fanfictions, Issei might find himself in a world where his devil powers are seen as something different or even feared in a new way. Maybe in a world of pure angels, he is the outcast and has to prove himself not to be an evil entity. This trope allows for a lot of character exploration and development for Issei.
The 'Alternate Universe' trope is very much used. For example, an alternate universe where the events of Unlimited Blade Works and Highschool DxD are intertwined from the start. Maybe the Grail War is a regular occurrence in the Highschool DxD world. Another trope is 'Identity Reveal'. A character might be hiding their true identity from one universe while in the other, and when it's revealed, it causes a stir. Also, the 'Forbidden Love' trope can be interesting. If there are rules against a relationship between a character from Unlimited Blade Works and one from Highschool DxD, like a magical or social taboo, it can create a lot of drama and tension in the fanfiction.
One of the popular tropes in Persona fanfiction is 'alternate reality'. Writers like to explore what would happen if the events in the game took a different turn. In Highschool DxD, the 'rivalry' trope is common, especially between different devil clans. In a fanfiction that combines both, a trope could be 'culture shock'. A Persona character coming into the Highschool DxD world would experience a huge culture shock due to the different rules, species, and social structures. And also, the 'mentor - mentee' trope could be used. Maybe a powerful character from Persona becomes a mentor to a young devil in Highschool DxD, guiding them through their power - up journey.
A third option could be 'Naruto and Highschool DxD: The Ghoul Invasion'. Here, the world of Naruto and Highschool DxD is suddenly invaded by ghouls. The story follows the heroes from both universes as they try to figure out the origin of the ghouls and how to stop them. It has a complex plot and well - developed characters.
Naruto himself is often very popular. His energetic and determined personality makes him stand out. He can bring his ninja skills and positive attitude into the Highschool DxD setting. Also, Issei from Highschool DxD is commonly included because of his central role in his own series and his unique relationship with the devils and other supernatural beings.
One popular crossover might be the exploration of the unique powers. For example, Naruto's jutsu in the Highschool DxD world could create interesting scenarios. His shadow clone jutsu could be used in battles against the devils or fallen angels in the Highschool DxD setting. Another aspect could be the character relationships. Naruto's friendly and determined nature could form unique bonds with the characters in Highschool DxD like Issei. This could lead to new storylines where they team up to face common enemies or help each other grow in power.
In Highschool DxD OC fanfiction, the 'Power - Up' trope is quite common. Your OC starts with a certain level of power and then, through training, battles, or special encounters, they gain more power and become stronger. The 'Redemption' trope can also be used. If your OC has made mistakes in the past or has a dark history, they can strive to redeem themselves throughout the story. Also, the 'Chosen One' trope works well. Your OC could be the one chosen to save the supernatural world from a great threat, like an impending war between devils, angels, and fallen angels. This gives them a sense of purpose and importance in the story.
Naruto is often a very popular character in these crossovers. His strong - willed nature and powerful ninja abilities make him stand out. He can bring a new level of action and adventure to the Highschool DxD setting.
Another common pairing is Sasuke and Akeno. Sasuke's brooding and powerful nature might attract Akeno, who is also quite powerful and has a bit of a mysterious side herself. They could bond over their shared power levels and a mutual understanding of what it means to be different from the norm in their respective worlds.